Performance testing of ultrasound Doppler equipment

Abstract: Blood and tissue velocities are measured and analysed in cardiac, vascular and other applications of diagnostic ultrasound. Errors in system performance might give invalid measurements.We developed two moving string test targets (“Doppler phantoms”) to characterise ultrasound Doppler systems. These phantoms were initially used to measure such variables as sample volume dimensions, location of the sample volume, and the performance of the spectral analysis. Specific tests were done to detect errors in signal processing causing time delays and inaccurate velocity estimation.Even time delays as short as 30 ms in cardiac motion pattern may have clinical relevance. These delays can be measured with echocardiography, by using techniques such as flow and tissue Doppler and M-mode together with external signals (e.g., ECG and phonocardiography). If one or more of these signals are delayed in relation to the other signals (asynchronous), an incorrect definition of cardiac time intervals can occur. To determine if this time delay in signal processing is a problem, we tested three commercial ultrasound systems. We used a digital ECG simulator and a Doppler string phantom to obtain test signals. We found time delays of up to 90 ms in one system, whereas delays were mostly short in the other two systems. Further, the time delays varied relative to system settings.To determine the accuracy in velocity calibration, we tested the same three ultrasound systems using the Doppler phantom to obtain test signals for flow and tissue pulsed Doppler and for continuous wave Doppler. The ultrasound systems were tested with settings and transducers commonly used in cardiac applications. In two systems the observed errors were mostly close to zero, whereas one system systematically overestimated velocity by an average of 4.6%. The detected errors can be considered small in clinical applications but might be serious in certain research applications. It is important to know the velocity error of the used ultrasound system and to judge it in relation to the application in which it is used.